1. Field of the Invention
The present invention relates to low viscosity dispersions comprising particulate material and a solvent.
2. Description of the Related Art
Pigments are finely divided, insoluble, solid particles which are, in general, not readily dispersible alone in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants.
The choice of dispersant depends on a variety of factors, including the morphological and chemical properties of the pigment and the type of liquid vehicle (resin or binder and solvent) in which the pigment is to be dispersed. However, the dispersant can often have a negative impact on properties such as color development and viscosity. This is particularly true at high pigment loading levels, such as greater than 10% pigment. For example, high surface area pigments are generally difficult to disperse without a high level of dispersant, but such a high level of dispersant often results in an unacceptable increase in the dispersion viscosity. Therefore, the amount of dispersant must be adjusted for the components used in order to obtain dispersion compositions with good overall properties.
Modified pigments, including modified colored pigments, have also been developed which provide dispersible pigment compositions, such as inks and coatings, with improved properties. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. The resulting surface-modified pigments can be used in a variety of applications, such as inks, inkjet inks, coatings, toners, plastics, rubbers, and the like. U.S. Pat. No. 5,885,335 describes the use of modified pigments having attached ionic and/or ionizable groups at specified levels in aqueous and non-aqueous ink and coating compositions. Also, U.S. Pat. Nos. 5,713,988 and 5,698,0916 describe modified pigments having various types of attached groups that can be used in non-aqueous inks and coatings. Furthermore, PCT International Publication No. WO 01/51566 describes a method of making a modified pigment by reacting a first chemical group and a second chemical group to form a pigment having attached a third chemical group. The first chemical group includes at least one nucleophile and the second chemical group includes at least one electrophile, or vice versa. This method may be used, for example, to prepare modified pigments having attached polymeric groups, which can be used in ink compositions and, in particular, inkjet ink compositions.
However, while these methods can be used to produce ionically modified pigments that can be dispersed in various types of non-aqueous compositions, it would be expected that these compositions would have high viscosity, particularly at high loading levels, in non-aqueous solvents. The reason for this is that the chargeable groups attached to the pigments would not be expected to be charged in a non-aqueous system and would therefore interact with each other, thereby increasing the viscosity. While such high viscosity systems may be useful in some applications, lower viscosity is generally more desirable since, for example, they are easier to process and can be applied using multiple techniques, including inkjet printing, dip coating, etc.
Therefore, while these methods provide dispersions of pigments, including modified pigments having attached groups, there remains a need for pigment dispersions, particularly those having high loadings of pigment, having improved performance properties, such as viscosity, thereby providing advantageous alternatives to previous modified pigments.